1. Field of the Invention
The present invention relates to a vibratory flow meter and method, and more particularly, to a vibratory flow meter and method for correcting for an entrained phase in a two-phase flow of a flow material.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
Flow meters are used to perform mass flow rate measurements for a wide variety of fluid flows. One area in which Coriolis flow meters can potentially be used is in the metering of oil and gas wells. The product of such wells can comprise a multiphase flow, including the oil or gas, but also including other components, including water and air, for example, and/or solids. It is highly desirable that the resulting metering be as accurate as possible, even for such multiphase flows.
Coriolis meters offer high accuracy for single phase flows. However, when a Coriolis flow meter is used to measure aerated fluids or fluids including entrained gas (emulsions), the accuracy of the meter can be significantly degraded. This is similarly true for entrained solids (slurries).
Entrained air is commonly present as bubbles in the flow material. The size of the bubbles can vary, depending on the amount of air present, the pressure of the flow material, and the temperature. The extent of the decrease in performance is not only related to how much total gas is present, but also to the size of the individual gas bubbles in the flow. The size of the bubbles affects the accuracy of the measurement.
One significant source of error is fluid decoupling. Fluid decoupling results from the motion of the gas bubbles with respect to the liquid as a result of the vibration of the tube. The relative motion of the gas bubbles with respect to the liquid is driven by a buoyant force that is similar to the force that causes bubbles to rise to the surface under the influence of gravity. However, in a vibrating tube, it is the acceleration of the vibrating tube that causes the bubbles to move and not the acceleration of gravity. Because the dense fluid resists the acceleration more strongly than the light bubbles, the bubbles are accelerated in the same direction as the tube acceleration. The bubbles thus move faster and further than the flow tube and the bubble motion causes some of the fluid to move more slowly than the flow tube. This is the basis of the decoupling problem. As a result, the fluid that has the lower vibrational amplitude undergoes less Coriolis acceleration and imparts less Coriolis force on the flow tube than it would in the absence of bubbles. This results in the flow rate and density characteristics being under-reported (negative flow and density errors) when entrained air is present.
Slurries present a problem similar to decoupling. In the case of slurries, however, the solid particles are often heavier than the liquid. Under the acceleration of the vibrating tube, the heavier particles move less than the liquid. This causes some of the liquid to move more than the vibrating tube. The result is that the liquid is over-reported (positive flow and density errors) when particles heavier than the liquid are present. In both cases, the entrained phase's differential motion is driven by the difference in density between the entrained phase and the liquid. If the compressibility of gasses is neglected, then the same equations can be used to describe the behavior of both entrained air and particles. Subtracting the entrained phase density from the liquid density gives positive numbers for gasses and negative numbers for solids. The decoupling of slurries is simply negative. For this reason the term decoupling will be used interchangeably for both emulsions and slurries.
Compensating for fluid decoupling has been difficult because there are several factors that determine how much the bubbles move with respect to the fluid. Fluid viscosity is an obvious factor. In a very viscous fluid, bubbles (or particles) are effectively frozen in place in the fluid and little flow error results.
Another influence on bubble mobility is the bubble size. The drag on a bubble is proportional to the surface area, whereas the buoyant force is proportional to the volume. Therefore, very small bubbles have a high drag to buoyancy ratio and tend to move with the fluid. Small bubbles subsequently cause small errors. Conversely, large bubbles tend not to move with the fluid and result in large errors. The same holds true for particles. Small particles tend to move with the fluid and cause small errors.
The density difference between the fluid and the gas is another factor. The buoyant force is proportional to the difference in density between the fluid and the gas. A high pressure gas can have a high enough density to affect the buoyant force and reduce the decoupling effect. In addition, large bubbles occupy more volume, leading to true fluctuations in the density of the flow material. Due to the compressibility of a gas, the bubbles can change in gas quantity and yet not necessarily change in size. Conversely, if the pressure changes, the bubble size can correspondingly change, expanding as the pressure drops or shrinking as the pressure increases. This can also cause variations in the natural or resonant frequency of the flow meter and thus variations in the actual two-phase density.
Second order factors also can have an effect on bubble and particle mobility. The turbulence in a high flow rate fluid breaks large bubbles and particles into smaller ones, thus reducing decoupling error. Surfactants reduce the surface tension of bubbles and decrease their tendency to coalesce. Valves can decrease bubble size through increased turbulence while pipeline elbows can increase bubble size by forcing them together through centrifugal force.
There remains a need in the art for a vibratory flow meter that detects problematic levels of entrained second-phase materials. There remains a need in the art for a vibratory flow meter that can accurately measure flow characteristics in the presence of entrained second-phase materials. There remains a need in the art for a vibratory flow meter that can accurately measure flow characteristics at varying levels of entrained second-phase materials.